I got it yesterday and today. But only when I tried starting two source mods, insurgency and d.i.p.r.i.p. It didnt occur when launching offline SP games like hl2. Interesting that only after trying launching the mods several times did it actually logged me off, and asked to login again. I logged in again, and it worked, didnt notice any problems either.

I've had this pop up on rare occasions, too. Seem to be legit.

Yeah, I think it's nothing to worry about.
Btw, this might be somewhat silly question, but I'm getting a new computer in coming weeks, but what happens to the steam-thingie? I've downloaded many games from my account to my current machine, can I download them also to the new computer? Won't my account show that I have the games already, or will it let me download them again?
Sorry if I'm typing a bit freakish, but it's night in Finland and I'm tired...

You will be able to install them on your new computer no problem. The games are shown to be installed if they are installed on that current computer, otherwise it's grayed out. You can theoretically install it on as many computers as you want, but you can only be online with Steam with one computer at a time (it will bump the other computer offline).

You can make backups of the installed games and put them in the new pc. THat way you dont have to download again.

Yep, I get this when I am dual booting (xp and win7) whenever I log in to one os, the other will give that warning, or from 1 system to the next (2 seperate computers).
As for games and new computers:
1. Copy the WHOLE steam folder somewhere.
2. Copy the steam folder to the new computer.
3. Install steam to wherever you have the old steam folder at.
4. Start steam.
5. Click your game, and select Install.
No need to re-download all your games.
Steam will check if it is correct, download any files necessary, and install the game again.

Thanks guys :)
And for the copying the steam folder....is that entirely...you know...."ok"?

Yes.

ok? What do you mean you can do whatever pleases you with it really, it 's you computer it's your files after all and lets just say that's a praised feature of STEAM to be available anywhere at anytime.
I would recommend only copying the "steamapps" folder minus your "username" folder, then you proceed to Install STEAM and copy the backup into your steam folder.
With the STEAM cloud thing I think the savegames of your Valve games get synchronized between your different computers so you would not need to backup them?

Thats only if the game supports it, i believe.